Photoacoustic waves are a type of elastic wave occurring during the process of thermoelasticity that takes place when a substance is irradiated with light in an absorption wavelength region. Therefore, photoacoustic waves have attracted attention as a method for imaging absorption properties. Photoacoustic waves are also a type of ultrasonic wave and are less easily affected by diffusion than light is. For these reasons, photoacoustic waves have been applied as a method of imaging the inside of an organism.
In the method used in a photoacoustic microscope for imaging that uses photoacoustic waves as a detection signal, pulsed light adjusted to the absorption wavelength region of the object under observation is used as excitation light, the excitation light is focused by an objective lens, the inside of the specimen is scanned by a focal spot, and the photoacoustic wave occurring at each focal spot as a result is detected with a transducer or the like. With such a photoacoustic microscope, when the specimen is scanned with the focal spot, a photoacoustic wave occurs when an absorbing substance is located at the focal spot position. Hence, by detecting the photoacoustic wave, the absorption properties within the specimen can be imaged.
JP 2011-519281 A (PTL 1), for example, discloses such a photoacoustic microscope. FIG. 18 illustrates the photoacoustic microscope disclosed in PTL 1. In FIG. 18, excitation light L from a non-illustrated laser pulsed light source passes through a focusing lens 101, pinhole 102, vibrating mirror 103, objective lens 104, correcting lens 105, isosceles prism 106, silicone oil layer 107, rhomboid prism 108, and acoustic lens 109 and is focused on the inside of a specimen S. The photoacoustic wave U generated at the focal light position within the specimen S by irradiation of the excitation light L undergoes wavefront conversion by the acoustic lens 109, is reflected within the rhomboid prism 108, and is detected by an ultrasonic transducer 110.
In FIG. 18, the isosceles prism 106 and the rhomboid prism 108 are joined with the silicone oil layer 107 therebetween. The acoustic lens 109 is joined to the rhomboid prism 108 so that the acoustic axis, which corresponds to the optical axis in an optical lens, matches the optical axis of the objective lens 104, and so that the acoustic wave focusing position of the acoustic lens 109 and the focal light position of the excitation light L within the specimen S match. The ultrasonic transducer 110 is joined to the rhomboid prism 108 so that the wavefront of the photoacoustic wave U from the focal point of the acoustic lens 109 is converted to a plane wave by the acoustic lens 109 and is perpendicularly incident on the detection surface of the ultrasonic transducer 110. The specimen S is immersed in liquid.